Dynamic pressure sensor, photo acoustic gas detector, microphone, hydrophone and method of their manufacture

ABSTRACT

The present invention pertains to a pressure sensor for measuring absolute dynamic pressure. The sensor comprises a frame surrounding a diaphragm, wherein the diaphragm is spaced from the frame by at least two slits which define a restriction that places a reference chamber in fluid communication with a surrounding environment. The diaphragm is connected to the frame at two transition areas, which transition areas are separated from one another by the slits.

BACKGROUND OF THE INVENTION

The present invention pertains to a pressure sensor for measuringabsolute dynamic pressure, and more particularly, a sensor comprising aframe, a diaphragm arranged in the frame and attached thereto alongparts of the outer edge of the diaphragm, where the diaphragm has ameasurement side toward the surroundings and a rear side, and further areference chamber behind the diaphragm rear side and a restrictionconnecting the reference chamber to the surroundings, as well as asignal-providing element arranged to detect mechanical stress in anattachment part in the outer diaphragm edge. The invention also pertainsto a photoacoustical gas detection sensor, a microphone and a hydrophonebased upon use of the pressure sensor. Further, the invention pertainsto a method for manufacturing a pressure sensor for dynamic absolutepressure, and a method for manufacturing a photoacoustical gas detectionsensor.

Pressure sensors based on movement/flexure of a diaphragm are as astarting point only able to measure differential pressures. E.g. insilicon pressure sensors the sensor element consists of a diaphragmwhich on respective sides thereof is in contact with a fluid underpressure, and which diaphragm flexes dependent on the pressuredifference between the fluids.

In order to manufacture an absolute pressure sensor with a differentialsensor as a starting point, one has to provide a substantially closedchamber with a reference fluid on one side of the diaphragm. The volumeof this chamber is called the reference volume. The reference volume,and consequently the reference pressure, will vary with temperature.Hence, temperature changes will introduce errors in the measurementsystem.

Another aspect that must be taken into consideration in connection withthe reference volume, is that when the diaphragm flexes toward thereference chamber, the fluid in the reference volume will be compressed,and thereby form a pressure, depending on the compressibility of thefluid. This phenomenon is termed pressure feedback, and such pressurefeedback affects the sensitivity of the system. In order to minimize thepressure feedback, the reference volume must be large, so that thediaphragm flexing volume constitutes only a small fraction of thereference volume. (Alternatively, the diaphragm must be rigid, and thismeans at the same time low sensitivity.)

One way of overcoming the two above mentioned problems of temperatureerrors and pressure feedback, is to arrange a reference chamber with avacuum behind the diaphragm. A temperature increase will then not giveany increase of pressure in the reference chamber, and pressure feedbackcannot arise. However, such an embodiment with a vacuum in the referencevolume, will lead to limitations in the pressure values that can bemeasured. If the static pressure is approximately 1 bar, as willnormally be the case in the surroundings, the diaphragm must bedimensioned to withstand a pressure difference of 1 bar if a vacuum isused in the reference volume, and such a diaphragm will be rather poorlysuited for measuring very small pressures, in particular dynamicpressures e.g. in connection with acoustical oscillations. When thesesmall, dynamic pressures are to be measured, the diaphragms must bedimensioned in relation thereto, and a static pressure difference ofe.g. 1 bar will then possibly lead to destruction of such a diaphragm.In other words, in connection with measuring dynamic pressures havingsmall pressure values, e.g. in the 1 pascal range, one must use areference volume that contains a fluid.

The sensitivity of the measurement system will also depend on the volumedisplacement of the diaphragm. The volume displacement is flexure volumeper pressure unit, i.e. the volume occupied by the flexed diaphragm inthe reference chamber, divided by the pressure. In general, a goodsensitivity implies a large flexure/volume displacement. This can easilybe appreciated by considering a very thin diaphragm that flexes easilywhen a pressure differential is present. The disadvantage of a largevolume displacement is that the pressure feedback increases, and thesensitivity of the sensor is reduced.

If it is desirable to measure rapidly changing, dynamic pressures,typically in connection with sound oscillations, it is possible to makea small channel into the reference chamber, thus letting the surroundingmedium into the chamber. At the outset this will make the diaphragm morerobust to exposure to atmospheric pressure, temperature changes, or tohandling. Such a channel or opening that connects the reference volumeto the sensor surroundings, or more generally to the pressure input ofthe sensor, is called a restriction. This is because the opening/channelis so narrow that it will take a long time until the pressure inside thereference volume is equalized in relation to the external pressure. Therestriction has the effect of a negative feedback regarding slowchanges, i.e. low frequencies in the pressure oscillations. The crosssection area and the length of the restriction represent a flowresistance, equivalent to an electrical resistance, and the referencevolume multiplied by the fluid compressibility, represents a reservoir,equivalent to an electrical capacity, and together the restriction andthe reference chamber then operate as a first order lowpass filter forthe pressure oscillations, since the low frequencies have sufficienttime to get through the restriction and thereby influence both sides ofthe diaphragm, while the high frequencies will merely affect thediaphragm side facing the surroundings/the pressure input, and thereforewill be measurable. In other words, the sensor will be sensitive tofrequencies higher than the corner frequency of this filter. By shapingthe restriction and the reference chamber in a suitable manner, it ispossible to control the corner frequency of the filter, and in this waythe measuring range of the sensor. If it is desirable to achieve anextended measuring range down toward low frequencies, then a largereference volume will be advantageous, since this provides a low cornerfrequency.

However, size is often an important criterion in manufacturing a sensorthat fulfils requirements set by an application. In order to make asmall sensor, it is of course important that the reference volume ismade as small as possible, but with a small reference volume, and whenit is desirable to measure relatively low frequencies, one must preparea very narrow restriction, and the diaphragm must give an extremelysmall volume displacement. Also if it is desirable to manufacturepressure sensors in planar technology, i.e. in batches, the size of thereference volume should be restricted. The advantage of this type ofmanufacturing is that the sensors can then be manufactured at areasonable cost.

The most common application regarding measurement of dynamic pressures,is in sound measurement, which comprises dynamic pressures all the waydown into the μPa range. When it is desirable to make measurements inthese pressure ranges, static pressure variations are quite destructive.Variations due to high and low pressures may amount to several tens ofkilopascals (1 kPa=10 mBar). If one wishes to measure dynamic pressuresunder water at various depths, the static pressure changes may be evenmuch larger. When dynamic pressures shall be measured, one will oftenuse sensor elements that are sensitive only to dynamic pressure, and themost common example of such a sensor element is a piezoelectric crystal.Such crystals have many good characteristics, inter alia a low price, ahigh natural frequency and a low sensitivity to acceleration. However,it is a disadvantage that such piezoelectric crystals have a limitedstability over time, as well as poor low frequency characteristics (incomparison with monocrystalline piezoresistive structures).

Therefore, one has lately to an increasing degree changed to usingdiaphragm sensors made by silicon, which material exhibits betterstability. Embodiments of such diaphragm sensors have been mentionedabove. One has tried to manufacture diaphragm sensors with restrictionsin order to provide a high sensitivity, and by means of cost reasonablemanufacturing technologies. For instance Norwegian patent applicationno. 97.1201 discloses a pressure sensor based on solid state technology,where e.g. a semiconductor chip, preferably with silicon as a startmaterial, is processed to comprise a relatively thick frame, anintermediate, thin diaphragm and a central, thick block that can bepushed down by an overpressure, a reference chamber being providedunderneath the block and diaphragm, between the semiconductor chip and asubstrate thereunder, e.g. a glass substrate. At least two rigid andthick beam connections between the central block and the surroundingframe provide areas where high mechanical stresses are induced when theblock exhibits a deflection due to a pressure variation, andsignal-providing piezoresistive elements are located in these areas,which in their turn are located where the beams pass on to the frame andthe block respectively. Pressure sensors of this type can be batchmanufactured from a larger semiconductor wafer that is bonded to alarger substrate disc, then to be cut into single sensors in the end.This previously known pressure sensor can also be equipped with arestriction to provide a connection between the reference chamber andthe sensor pressure input, so that a suitable corner frequency can beprovided for the low end of the frequency measuring range. However, thisknown sensor has disadvantages like a large area and a significantvolume displacement due to the thin diaphragm areas, and these featuresdo not contribute to the sensitivity. Besides, the manufacturing processis relatively complicated and costly, and a sensor with a closedreference chamber is not very suitable for batch production.

Norwegian patent no. 300,078 discloses a photoacoustical gas detectorhaving a chamber that contains a gas type to be detected somewhere else.The chamber has been manufactured by bonding together two silicon orquartz plate elements prepared by the use of planar technology. Thechamber has windows for transmission of pulsed IR radiation, and apressure sensor with a diaphragm is arranged above a closed space thatcommunicates with the chamber. However, this gas detector exhibits clearlimitations regarding practical implementation and manufacturing costs.The location of a signal-providing element in relation to the diaphragm,in order to achieve high sensitivity, is not mentioned in the patent.Nor are any restrictions between a measurement chamber and a referencechamber mentioned in that publication.

U.S. Pat. No. 5,633,552 discloses a pressure sensor which comprises achamber and a slab attached to a frame lying on top of the chamber. Theslab is attached to the frame along one of the sides, and constitutessort of a cantiliever beam from the frame. The remaining three sides ofthe slab are separated from the frame by a vertical slit. Apiezoelectric element is placed on the slab in the area where it isattached to the frame. The slit width is approximately 10 μm. Thisdevice has a high sensitivity (2 mV/μBar for frequencies situated in therange 100-1000 Hz), the slab being easily movable by means of smallpressure differences. In order to maintain the slab flat in the restingposition, it is manufactured in the form of three sandwiched layershaving different internal stresses. The device has a relatively largevolume displacement, and therefore large pressure feedback. In additionthereto, the use of a piezoelectric element for measuring mechanicalstress will lead to unstable measurements, since a piezoelectric elementis not as stable over time and with regard to temperature, as e.g. apiezoresistive element. The sensor indicated in the publication must beprovided with a relatively large reference chamber in order tocompensate for the effect of the large volume displacement. This makesbatch manufacturing of the complete sensor very difficult. The top partof the sensor, i.e. the sides and roof (slab) of the reference chamberare batch manufactured as parts of a larger wafer, but thereafter thewafer, including chamber sides and slabs, is cut into a plurality of topparts, and each respective top part is laminated to a bottom. Thismanufacturing method is costly and not very efficient.

The purpose of the present invention is in a first aspect to provide apressure sensor for measuring absolute, dynamic pressure, which sensorsatisfies high sensitivity requirements, and which sensor is providedwith a small reference chamber, and furthermore, it can be manufacturedin planar technology to provide a reasonable manufacturing cost. Otheraspects of the invention will appear below.

SUMMARY OF THE INVENTION

In accordance with the present invention, the purpose is achieved by apressure sensor of the type indicated in the introduction, and which ischaracterized in that the diaphragm is attached to the frame along atleast two parts of the outer diaphragm edge, at least one of thediaphragm attachment parts comprising an area where mechanical stressescaused by the pressure, are concentrated, the signal-providing elementbeing arranged in this area, and that remaining parts of the outer edgeof the diaphragm are separated from the frame by slits constituting therestriction.

In an important embodiment, the diaphragm and the frame are in one pieceformed from a planar material disk, the slits then being slits all theway through the disk thickness, said area then being a transition areafrom the diaphragm to the frame.

The measurement side of the diaphragm is preferably in the same plane asthe adjacent surface of the frame.

In a preferred embodiment, the material disk is of silicon, and thesignal-providing element is then preferably a monocrystallinepiezoresistive element.

In a further preferred embodiment, the slits that as a starting pointhave a depth dimension substantially transverse to the measuring side ofthe diaphragm, continue in under the rear side of the diaphragm, theframe or the wall of the reference chamber being shaped with a shoulderclosely adjacent the rear side of the diaphragm in an area along thediaphragm edge.

Preferably, the dimensions of the restriction are adapted to provide aflow resistance that together with the volume of the reference chamberconstitutes a low pass filter for the pressure equalising rate of thesensor, said filter having a corner frequency adapted to the frequencyrange of the dynamic pressure variations to be measured.

Several diaphragm attachment parts may be formed, arranged withintervals along the outer edge of the diaphragm, and these attachmentparts will comprise stress concentration areas, as mentioned above.

Preferably, the sensor comprises at least one piezoresistive elementincorporated into said area or said areas, as a signal-providingelement, preferably as an element in bulk material or as a thin filmelement on the surface.

In another aspect of the invention, there is provided a photoacousticalgas detection sensor for detection and/or concentration determination ofa particular gas in the surroundings of the sensor, wherein the sensorcomprises a measurement chamber filled with the gas in question, aradiation source spaced from the measurement chamber, and inside oradjacent to the measurement chamber a pressure sensor for measuringabsolute dynamic pressure in the measurement chamber. The measurementchamber is equipped with at least one wall area or window that istransparent to radiation from the source with a wavelength that can beabsorbed or scattered by the gas to be detected. The pressure sensorcomprises a frame, a diaphragm arranged in the frame and attachedthereto by the outer diaphragm edge, said diaphragm having a measurementside toward the measurement chamber and a rear side. Further, thepressure sensor comprises a reference chamber that substantially is notexposed to radiation, behind the rear side of the diaphragm, and arestriction connecting the reference chamber to the measurement chamber,and a signal-providing element arranged to detect mechanical stress inan attachment part at the outer diaphragm edge. The gas detection sensoris characterized in that the diaphragm is attached to the frame along atleast two parts of the outer diaphragm edge, at least one of theattachment parts of the diaphragm comprising an area in which mechanicalstresses caused by the pressure, are concentrated, and that remainingparts of the outer edge of the diaphragm are separated from the frame byslits constituting the restriction, the reference chamber and themeasurement chamber constituting together a closed system filled by thesame gas.

The measurement chamber and the reference chamber of the gas detectionsensor are manufactured by laminating and etching techniques and withsubstantially similar dimensions on respective sides of the frame withthe diaphragm and the slits, in order to constitute a compact unit.

In a preferable embodiment of the gas detection sensor in accordancewith the invention, the diaphragm has a light reflecting coating on oneof its sides, a wall area of the measurement chamber opposite to thediaphragm then having a light transparent area/window.

In a further aspect of the invention, there is provided a microphonecomprising a pressure sensor for measuring absolute dynamic pressure, amicrophone housing and signal wires, and the microphone is characterizedin that the pressure sensor is of the type indicated in the first aspectof the invention.

In a further aspect of the invention, there is provided a hydrophonecomprising a pressure sensor for measuring absolute dynamic pressure, ahydrophone housing and signal wires, and the hydrophone in accordancewith the invention is characterized in that the pressure sensor is ofthe type indicated in the above first aspect of the invention.

A further aspect of the invention comprises a method for manufacturing apressure sensor for dynamic absolute pressure, of the type indicated inthe first aspect of the invention, and wherein the diaphragm and theframe are in one piece formed from a plane material disk, the slits thenbeing slits all the way through the disk thickness, so that thementioned area is then a transition area from the diaphragm to theframe. The method in accordance with the invention is characterized inthat in a first, thin disk, preferably of silicon, a restriction isprovided in the form of at least two elongate, narrow slits, preferablyby means of ionic etching, said restriction defining a diaphragm inrelation to a surrounding frame in that the restriction substantiallysurrounds the outer edge of the diaphragm, except along transition areasfrom the diaphragm to the frame,

that in a second material disk, preferably of silicon, a recess isetched out, e.g. by wet etching, and

that the two disks are laminated to each other so that the recessconstitutes a reference chamber behind the diaphragm, and so that therestriction leads into the reference chamber.

In a first embodiment of the method, the second material disk, prior tolaminating to the first disk, is exposed to a further etching step withprecise forming of a step along the upper edge of the recess, in such aposition that when the laminating is made, the restriction is providedwith a bent extension in behind the diaphragm.

In a second embodiment of the method, a third material layer is providedbetween the two disks by depositing a thin film on to one of the disks,by growing e.g. an oxide on one of the disks or by laminating a thirdmaterial disk to one of the disks, with an opening adapted to and notsmaller than the opening in the frame, and thereafter the remaining oneof the two disks is laminated to the disk having the third materiallayer, so that the restriction is provided with a bent extension inbehind the diaphragm by means of the third material layer and the edgeof the second disk around the recess.

In a third embodiment of the method in accordance with the invention,the first material disk, prior to laminating to the second disk, isexposed to a further etching step on its rear side, at least in an areaaround the rear side opening of the restriction, in order to provide therestriction with a bent extension in behind the diaphragm when thelaminating is made.

In a favorable embodiment of the method, a number of sensors are batchmanufactured by preparing a number of first disks simultaneously andnext to each other on a first, larger material disk, a number of seconddisks are prepared in a corresponding manner next to each other on asecond, larger material disk, the two larger material disks arelaminated to each other, and single sensors are then provided bysectioning the bonded, larger disks.

In a further aspect of the invention there is provided a method forproducing a photoacoustical gas detection sensor of the type indicatedaccording to the second aspect of the invention, and this method ischaracterized in

that in a first disk, preferably of silicon, a restriction is providedhaving the shape of at least two elongate, narrow slits, preferably bymeans of ionic etching, said restriction defining a diaphragm inrelation to a surrounding frame in that the restriction substantiallysurrounds the outer edge of the diaphragm, except along transition areasfrom the diaphragm to the frame,

that in a second disk, preferably of silicon, a first recess isprovided, e.g. by machining or wet etching,

that the two disks are laminated to each other in order that the firstrecess shall constitute a measurement chamber in front of the diaphragm,and in such a manner that the restriction leads into the measurementchamber,

that in a third disk, preferably of glass, there is provided a secondrecess, e.g. by machining or wet etching, and

that the third disk is laminated to the first disk in an atmosphere of apredetermined gas in such a manner that the second recess constitutes areference chamber behind the diaphragm, whereby the restriction alsoleads into the reference chamber and thereby constitutes a channelbetween the measurement chamber and the reference chamber, whichchambers will both contain this gas after the lamination.

In a favorable embodiment of the method for producing a photoacousticalgas detection sensor, the second disk is provided with a through hole,e.g. by machining, and a fourth disk that is transparent to light whichexcites the gas in the measurement chamber, is laminated to the seconddisk to cover the hole.

In a further favorable embodiment, a number of gas detection sensors arebatch manufactured by preparing groups of the first, second, third andpossibly fourth disks respectively as parts of larger first, second,third and possibly fourth material disks respectively, the respectivelarger material disks are laminated to each other, possibly in anatmosphere of a certain gas that is to be detected, and single gasdetection sensors are then provided by dividing the bonded larger disks.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention shall be described in further detailwith reference to exemplary embodiments, and with reference to theappended drawings, where

FIG. 1 shows schematically a general pressure sensor for absolutepressure,

FIG. 2a and FIG. 2b show two possible embodiments of the diaphragm inthe sensor in accordance with the invention,

FIG. 3 shows a section through the embodiment appearing in FIG. 2b,

FIG. 4 shows an embodiment of a gas detection sensor in accordance withthe invention, and

FIG. 5 shows an alternative embodiment of a gas detection sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 appears an absolute pressure sensor according to the priorart. The sensor comprises a reference chamber 1, a sensor diaphragm 2and a restriction 3. In the shown embodiment the restriction consistssimply of openings in diaphragm 2. Because of the restriction 3, thereference chamber 1 will, as previously mentioned, necessarily containthe same gas as the surroundings, and have the same static pressure asthe surroundings. For dynamic pressures with oscillation frequencieshigher than a typical corner frequency determined by the restriction 3and characteristics of the gas inside the chamber 1, the diaphragm willexhibit a useful sensitivity, and a measurement signal can be achievedfrom the sensor, using a signal-providing element associated with thediaphragm for measuring the deflection thereof, or possibly via someother parameter associated with the deflection.

A fundamental quality of diaphragm sensors is that the sensitivity isdependent on the volume displacement. The simplest manner forvisualizing this effect, is to consider the energy that is introducedinto the elastic diaphragm when it is affected by a pressure. For agiven pressure, the work done by the pressure, is equal to W=PV, where Vis the diaphragm volume displacement. This must be the same energy thatis stored in the mechanical stresses in the diaphragm (a very smallenergy storage in compressing the inside fluid is here disregarded),given by the expression W=∫∫∫_(v)σ^(dv/)2E, where v is the volume ofmaterial (e.g. silicon crystal) containing the stresses, E is themodulus of elasticity of the material and σ is the mechanical stress.One can see here that if sensitivity shall be increased withoutincreasing the volume displacement, the stresses must be concentratedwithin a diaphragm area that is as small as possible.

As regards the diaphragm in a sensor in accordance with the invention,one utilizes just the feature of concentrating stresses within a smallarea of the diaphragm, and more specifically, this is done by providinglimited connection areas between the diaphragm and the frame surroundingthe diaphragm. This feature then leads to high sensitivity, even whenusing a small reference volume.

In FIG. 2a is shown an embodiment of the diaphragm in a sensor inaccordance with the invention. The diaphragm 2 is viewed “from above”,i.e. in the direction in which pressure works against the diaphragmsurface. The restriction 3 is shaped as two slits all the way throughthe material which initially forms a complete and connected chip, whichhowever by means of the slits 3 is separated into a surrounding frame 4and the diaphragm 2. The diaphragm is only attached to the frame attransition areas 5 such that the transition areas 5 serve as attachmentstructures that connect the diaphragm 2 to the frame 4. The mechanicalstresses caused by the pressure on the diaphragm 2, are concentrated inthese transition areas.

In FIG. 2b appears a different configuration, however the respectivereference numerals refer to corresponding details as in FIG. 2a. Acorresponding concentration of stresses is achieved also in this case,in the areas having reference numeral 5.

In FIG. 3 appears a section through the sensor shown schematically inFIG. 2b, indicated by means of a broken line. In FIG. 3 appears inparticular an embodiment with an extension of the restriction 3, in theform of an extended channel 6 that is bent in under the diaphragm 2. Thepurpose of this restriction extension is to provide a furtherpossibility for regulating down the corner frequency of the filter. Thissolution is provided to give a sufficiently large flow resistance whenthe fluid is a gas. It is a fact that it is not certain that the presenttechnology is able to produce sufficiently narrow slits directly, and anextension of the restriction such as shown in FIG. 3, will then possiblyremedy this.

Of course, other geometries for such slits 3 that are an essentialfeature of the invention, can be provided. The geometries shown in FIG.2a and FIG. 2b are only examples. The important point is the combinationof a restriction and a concentration of stresses in a small area.Etching of thin slits all the way through the material chip, thereby toprovide a diaphragm, will then give just the desired combination. Thesystem will operate approximately like a piston that is fastened bymeans of a few small springs.

Regarding the embodiment shown in FIG. 3, it is to be noted that alsothe geometry as viewed in a section through the diaphragm and underlyinglayers, will be variable within the inventive framework. The materialchip constituting the top, i.e. the diaphragm and the frame, needs e.g.not have the same thickness all over, see for instance FIG. 5. Theshaping of the reference chamber 1 can be made in several ways, andthere are also different manners for providing the extra extension ofthe restriction.

In one embodiment the diaphragm may be equipped with several areas forstress concentration, i.e. areas where the diaphragm is attached to theframe. This configuration gives a more stable sensor, and diminishes thevolume displacement.

If the restriction or slits 3 are shaped to be very narrow, i.e. narrowin relation to the height of the restriction (which height in turncorresponds to the thickness of the diaphragm), one enables achievementof good flow resistance characteristics, and consequently a low cornerfrequency, which is desirable. With dry etch techniques like e.g. RIE(Reactive Ion Etching) and ICP (Inductively Coupled Plasma), it ispossible to etch narrow slits all the way through silicon diaphragms.Even with a silicon diaphragm as thin as 50 μm, it will be possible withthis type of etching to provide slits 3 which in the section shown inFIG. 3 will have a height to width ratio of 30:1.

Noting that the idea in its most general form is not limited to planartechnology or use of silicon as main material in diaphragm, frame andpossible underlying layers, it is remarked that silicon planartechnology can be used to manufacture sensors in an efficient andreasonable manner. In this type of technology it is e.g. possible toproduce sensors that can do with a reference volume as small as e.g. 800μm×3,5 mm×3,5 mm, which is a volume that can be implemented in a simplemanner with planar technology. Thus, the sensor can be batchmanufactured. To compensate for the small reference volume, the volumedisplacement is limited correspondingly (for instance to 10⁻¹⁴ m³/Pa orless).

As previously mentioned, an extended channel 6 can be made, whichchannel is parallel to the diaphragm 2 and is connected to the lower endof the slits 3, i.e. the end closest to the reference chamber 1. Thisadditional channel 6 can be provided by e.g. fixing another silicon chipby laminating, in which other silicon chip there has been processed,e.g. etched, a step in the surface thereof in advance. Such step can bemade small and precise, maybe with a dimension smaller than 0.1 μm, andwith a variation less than 0.01 μm. Other ways of providing the extendedchannel shall be mentioned later. Using the extended channel 6, i.e. anextension of the restriction 3, corner frequencies as low as 1 Hz can beachieved, and consequently, measuring ranges can be achieved that arewider than the measuring ranges in the prior art solutions.

The sensor of the invention also comprises a signal-providing element,and in a favorable embodiment of the invention one uses a piezorsistiveelement that is incorporated into the diaphragm, or more particularlyinto the transition area 5 between the diaphragm 2 and the frame 4, vizin a position where the mechanical stresses are at their highest. Suchpiezoresistive elements can be made within the framework of planartechnology, and therefore are favorable signal-providing elements. Apiezoresistive Wheatstone measuring bridge provides the opportunity forcoherent detection of dynamic pressure. This is because the signal fromthe measuring bridge is the product of the pressure and the feedvoltage.

“Planar technology” is here intended to mean a manufacturing technologyknown from semiconductor manufacture, one may mention epitaxial growth,vapor deposition, sputtering, photolithographic techniques using maskingand etching, oxidation processes, diffusion processes as well asmicromachining techniques like wet etching, dry etching, waferlaminating etc., all well known techniques which do not need moredetailed mention herein.

An obvious use for a sensor in accordance with the invention, is inmicrophones, and an aspect of the idea relates to just a microphonewhere a sensor of the type mentioned constitutes the central element.With a suitable microphone housing and necessary wire connections, themicrophone is complete, and further description of a microphone inaccordance with this invention, is not necessary. Low frequencylimitation of the microphone is given by the parameters mentioned above.

Correspondingly, one may manufacture a hydrophone in accordance with theinvention, using corresponding considerations as for a microphone.However, in the hydrophone case it is not very probable that sensors arebatch manufactured, because the compressibility of liquids is too low,and therefore it is necessary to use a larger reference volume.

FIG. 4 shows an embodiment of a photoacoustical gas sensor in accordancewith the second aspect of the invention. One can see that the differencein relation to what is shown in FIG. 3, is essentially that chambers 1and 7 are formed both on top and on the underside of the diaphragm 2, sothat the diaphragm 2 constitutes a division between a reference chamber1 and a measurement chamber 7. The two chambers 1 and 7 contain aspecial gas, and the gas sensor is then adapted to detect or measure thepressure of such a gas in the surroundings or in another volume. Aradiation source is used for irradiation of the surroundings or thevolume to be investigated, with a wide spectrum of wavelengths thatcontain strong absorption lines for the gas in question. The measurementchamber 7 is arranged so that the radiation enters the measurementchamber after transmission through the volume to be measured. Thedisc/chip that defines the reference chamber 1, may well be constitutedof glass, in order that the laminating technique shall be compatiblewith metallized discs/chips and filling of most gas types. Glass is nottransparent to radiation that typically excites gases (infraredradiation), and therefore cannot be used for the measurement chamber.Silicon is however transparent to a plurality of wave-lengths that canexcite gases, but not to all such wavelengths. For certain gases, e.g.germanium may constitute a better window. If no gas of the type inquestion is present in the volume to be investigated, the radiation willpass unhindered, enter the measurement chamber 7 and heat the gas in themeasurement chamber, because just this wavelength is absorbed by thisgas. Using pulsing of the radiation from the radiation source, pressureoscillations will arise in the chamber 7, and the diaphragm 2 will pickup these oscillations.

If the gas in question is found in the investigation area, part of theradiation will be absorbed and scattered, less radiation will reach themeasurement chamber 7, and the influence on the diaphragm 2 will beweaker. Thereby, a weaker sound signal is obtained from the gas sensor.

In a favorable embodiment of such a photoacoustical gas sensor, thediaphragm 2 comprises a light reflecting coating, whereby the lighttransmission in the chamber is doubled, which provides an increasedsensitivity, and furthermore it is ensured that light does not enter thereference chamber 1.

One sees that in principle reference chamber 1 and measurement chamber 7might equally well be interchanged as to their function since therestriction 3 and extended channel 6 work regardless of which chamberconstitutes the reference chamber and the measurement chamber. Thus, ifthe volumes of chambers 1 and 7 are of the same size, either one ofthese chambers can function as the measurement chamber or the referencechamber. Which materials are used on the different sides of thediaphragm, is then also connected with which gases shall beinvestigated, and which wavelengths are suitable for special gases.

It is to be noted that the gas sensor can be changed into a pressuresensor such as mentioned above, by providing an opening to thesurroundings as indicated e.g. in broken lines right below measurementchamber 7.

In FIG. 5 is shown an alternative embodiment of a photoacoustical gassensor in accordance with the invention. The same reference numerals areused for the same parts as in FIG. 4, it can be noted that the lowerlaminated silicon layer that provides a measurement chamber 7 togetherwith a relatively thick, upper silicon layer, equally well can bereplaced by e.g. a layer of germanium or some other suitable material.In the embodiment shown it appears that the diaphragm 2 itself is etcheddown to a quite different thickness from the starting material waferthat constitutes the frame 4 outside the diaphragm 2. (Etching down thestarting material wafer/chip enables larger flexibility regarding choiceof diaphragm thickness. Generally one does not wish to limit diaphragmthickness to standard starting material thickness in low costapplications.) For the rest, in this embodiment one sees that nohorizontal restriction part such as shown with reference numeral 6,inFIG. 4, has been used.

In order to change the gas sensor shown here into a pressure sensor, itis possible either to make an opening as shown in broken lines at thebottom, or the complete lower silicon layer can be removed.

When a pressure sensor in accordance with the invention shall bemanufactured by means of planar technology, initially a first, thin discis provided, preferably from silicon. By means of ionic etching oranother corresponding technique, the restriction 3 mentioned above isprovided in order to define a diaphragm in relation to a surroundingframe in the initial disc/wafer/chip. In a second material disc, alsopreferably made from silicon, however not limited thereto, a recess ismade, e.g. by wet etching or machining, and thereafter the two discs arelaminated to each other so that the recess in the second material discconstitutes a chamber behind the diaphragm, and so that the restrictionleads into the chamber. This chamber will constitute the referencechamber of the sensor.

In a preferred embodiment of the invention, a transverse extension ofthe restriction shall be provided on the underside of the diaphragm. Oneway of doing this has already been mentioned, namely an additionaletching step along the upper edge of the recess that is formed in thesecond material disc, so as to form thereby a bent extension such asshown by reference numeral 6 in FIG. 3, when the two material discs arelaminated together.

“Laminating” is in this case intended to comprise techniques for bondingdiscs together. For silicon this concerns e.g. field-assistedlaminating, thermal laminating, eutectical laminating (soldering),gluing etc.

An alternative way of forming a corresponding restriction extension 6 isby providing a third material layer between the two discs.

This may take place by depositing a thin film on one of the two materialdiscs, growing an oxide on one of the discs, or laminating a thirdmaterial disc to one of the discs. Regardless, the third material layermust be formed with an opening that is adapted to and not smaller thanthe opening in the frame. The remaining one of the two discs isthereafter laminated to the disc that has received the third materiallayer, and thereby the desired restriction extension 6, is obtained inbehind the diaphragm by means of the third material layer and the edgeof the second disk around the recess.

In a third, alternative method the first material disc is exposed to afurther etching step on its rear side/underside, prior to laminating tothe second disc, at least in an area surrounding the rear side openingof the restriction in the diaphragm. This also results in a bentrestriction extension in behind the diaphragm when laminating is madebetween the first and the second material discs.

When using planar technology, the reduced dimensions allow, ifdesirable, that a large group of sensors are batch produced, i.e. theetching processes, laminating processes etc. are executed on largermaterial wafers that comprise many areas that are processed to be sensorelements. When the wafers finally are laminated together, single sensorscan be sawn or broken apart. Such a manufacturing technique leads to lowproduction costs.

Regarding manufacturing photoacoustical gas sensors, correspondingtechniques as just mentioned, are used, but in this case an additionallayer shall also be laminated to the opposite side of thediaphragm/frame, for providing a measurement chamber. In thisconnection, gas filling of both chambers (measurement chamber andreference chamber) is effected at the same time during laminating thelast disc, which laminating has the consequence of closing the system.This last lamination should preferably be borosilicate glass againstsilicon, so that field-assisted laminating is possible. Field-assistedlaminating is made at a relatively low temperature, and will beunproblematic for most gases. Furthermore, it is to be noted that thethird disc (in essence after the first disc which defines diaphragm andframe, and the second disc that defines a measurement chamber) has thefunction of defining a reference chamber, and is provided with a specialrecess by means of machining or wet etching, before the above mentionedlaminating in an atmosphere of the predetermined gas. The third disc maypreferably be constituted of glass, or at least of a material that isopaque to the light wavelength in question. The disc that defines ameasurement chamber may possibly be provided with a hole, whereafter anadditional disc that is transparent to light that excites the gas in thechamber, is laminated to the disc with the hole in order to cover thehole, whereby light can enter the chamber as desired.

In a corresponding manner as in the batch manufacture of pressuresensors, also photoacoustical gas sensors in accordance with theinvention can be batch manufactured.

The disclosed embodiments have been described in order to explainvarious aspects of the invention, but shall not be regarded to limit theinvention in any way. As mentioned, sensors can be provided where alldiaphragm edges have stress concentration areas, or sensors where anedge may have two or more stress concentration areas. The materials thatare used, are not restricted to the ones that have been mentioned.Numerous semiconductor materials can be used in planar technology, andalso other materials may be used.

As previously mentioned, the reference chamber and the measurementchamber in a gas detection sensor can be interchanged, and restrictionextensions can be provided both “above” and “below” the diaphragm. Thediaphragm may have the same thickness as the material disk itconstitutes part of, or it may be etched down to another thickness.Substrate disks providing the reference chamber, can be produced usingone single disk, or disks laminated together in several layers. Lightreflecting layers on a diaphragm may be located on any side of thediaphragm. The slits 3 do not need to be “perpendicularly” cut throughthe diaphragm/frame, but may equally well exhibit a bent/angled shape,as seen in a cross section like FIG. 3.

What is claimed is:
 1. A pressure sensor for measuring absolute dynamicpressure, comprising: a frame; a reference chamber; a diaphragm attachedto said frame, with said diaphragm comprising a thin planar structurehaving an outer edge, a measurement side that is to face a surroundingenvironment, and a rear side facing said reference chamber; a firstattachment structure interconnecting said frame and a first section ofsaid outer edge of said diaphragm; a second attachment structureinterconnecting said frame and a second section of said outer edge ofsaid diaphragm; a restriction that is to interconnect said referencechamber and the surrounding environment such that said reference chamberis in fluid communication with the surrounding environment; and asignal-generating element to detect mechanical stress in at least one ofsaid first attachment structure and said second attachment structure,wherein at least one of said first attachment structure and said secondattachment structure provides a reduced area where mechanical stressescaused by pressure are concentrated, and said signal-generating elementis arranged at said reduced area, and wherein sections of said outeredge of said diaphragm other than said first section and said secondsection are separated from said frame by slits, with a combination ofsaid slits constituting said restriction.
 2. The pressure sensoraccording to claim 1, wherein said frame and said diaphragm are formedfrom a single sheet of material, said slits extend completely throughsaid sheet of material, and said reduced area comprises an area of saidsheet of material that transitions from said diaphragm to said frame. 3.The pressure sensor according to claim 1, wherein said measurement sideof said diaphragm is co-planar with a front surface of said frame. 4.The pressure sensor according to claim 2, wherein said sheet of materialcomprises a sheet of silicon material, and said signal-generatingelement comprises a monocrystalline piezoresistive element.
 5. Thepressure sensor according to claim 1, wherein said slits have a firstportion that extends in a depth direction that is substantiallytransverse to said measuring side of said diaphragm and a second portionthat extends under said rear side of said diaphragm, and one of saidframe and a wall of said reference chamber has a shoulder that isclosely adjacent said rear side of said diaphragm and along said outeredge of said diaphragm, with said shoulder cooperating with at least oneof said rear side of said diaphragm and a rear surface of said frame todefine said second portion of said slits.
 6. The pressure sensoraccording to claim 1, wherein dimensions of said restriction areselected to provide a flow resistance that together with a volume ofsaid reference chamber constitute a low pass filter for a pressureequalizing rate of the pressure sensor, with said low pass filter havinga corner frequency that is adapted to a frequency range of dynamicpressure variations to be measured.
 7. The pressure sensor according toclaim 1, wherein each of said first attachment structure and said secondattachment structure provides a reduced area where mechanical stressescaused by pressure are concentrated, and said first attachment structureis spaced from said second attachment structure along said outer edge ofsaid diaphragm.
 8. The pressure sensor according to claim 1, whereinsaid signal-generating element comprises a piezoresistive element, withsaid piezoresistive element being one of a bulk material element and athin film element.
 9. A photoacoustical gas detection sensor fordetection and/or concentration determination of a particular gas in asurrounding environment, comprising: a measurement chamber filled withan amount of said particular gas; a radiation source that is spaced fromsaid measurement chamber, with said measurement chamber having a wallarea or window that is transparent to radiation of said radiation sourcethat has a wavelength that can be one of absorbed and scattered by saidparticular gas; and a pressure sensor to measure absolute dynamicpressure in said measurement chamber, with said pressure sensor beingone of inside said measurement chamber and adjacent said measurementchamber and including (i) a frame; (ii) a reference chamber that is notto be exposed to the radiation of said radiation source; (iii) adiaphragm attached to said frame, with said diaphragm including a thinplanar structure having an outer edge, a measurement side facing saidmeasurement chamber, and a rear side facing said reference chamber; (iv)a first attachment structure interconnecting said frame and a firstsection of said outer edge of said diaphragm; (v) a second attachmentstructure interconnecting said frame and a second section of said outeredge of said diaphragm; (vi) a restriction that interconnects saidreference chamber and said measurement chamber such that said referencechamber is in fluid communication with said measurement chamber; and(vii) a signal-generating element to detect mechanical stress in atleast one of said first attachment structure and said second attachmentstructure, wherein at least one of said first attachment structure andsaid second attachment structure provides a reduced area wheremechanical stresses caused by pressure are concentrated, and saidsignal-generating element is arranged at said reduced area, whereinsections of said outer edge of said diaphragm other than said firstsection and said second section are separated from said frame by slits,with a combination of said slits constituting said restriction, andwherein said reference chamber and said measurement chamber constitute aclosed system filled with said particular gas.
 10. The photoacousticalgas detection sensor according to claim 9, wherein said measurementchamber and said reference chamber are each formed by performinglaminating and etching techniques and have substantially similardimensions on respective sides of said frame and diaphragm.
 11. Thephotoacoustical gas detection sensor according to claim 10, wherein aradiation reflective coating is provided on one side of said diaphragm,and a wall area of said measurement chamber that is opposite to saidradiation reflective coating includes a radiation transparent area orwindow.
 12. A method for manufacturing a pressure sensor for measuringabsolute dynamic pressure, comprising: forming at least two elongatenarrow slits completely through a thin first sheet of material such thatsaid slits define a restriction and divide said first sheet of materialinto (i) a frame, (ii) a diaphragm attached to said frame and separatedfrom said frame by said at least two slits, with said diaphragm havingan outer edge, a measurement side that is to face a surroundingenvironment, and a rear side, (iii) a first attachment structureinterconnecting said frame and a first section of said outer edge ofsaid diaphragm, and (iv) a second attachment structure interconnectingsaid frame and a second section of said outer edge of said diaphragm;forming a recess in a second sheet of material; laminating said firstsheet of material to said second sheet of material such that said recessdefines a reference chamber that faces said rear side of said diaphragmand is in fluid communication with said restriction, whereby saidrestriction places said reference chamber and the surroundingenvironment in fluid communication with one another when saidrestriction is placed in fluid communication with the surroundingenvironment; and providing a signal-generating element to detectmechanical stress in at least one of said first attachment structure andsaid second attachment structure, wherein at least one of said firstattachment structure and said second attachment structure provides areduced area which corresponds to an area of said first sheet ofmaterial that transitions from said diaphragm to said frame wheremechanical stresses caused by pressure are concentrated, and theprovision of said signal-generating element includes arranging saidsignal-generating element at said reduced area.
 13. The method accordingto claim 12, wherein said first sheet of material and said second sheetof material each comprise a sheet of silicon material, the formation ofsaid at least two slits in said first sheet of material comprises ionicetching said first sheet of material, and the formation of said recessin said second sheet of material comprises wet etching said second sheetof material.
 14. The method according to claim 13, further comprisingprior to the lamination of said first and second sheets of material,etching said second sheet of material to form a step along an upper edgeof said recess such that after the lamination of said first and secondsheets of material said restriction is defined by a first portion ofsaid at least two slits that extends through said first sheet ofmaterial and a second portion that extends under said rear side of saiddiaphragm.
 15. The method according to claim 13, further comprisingdepositing a thin film onto one of said first and second sheets ofmaterial by performing one of (i) growing an oxide layer on said one ofsaid first and second sheets of material, and (ii) laminating a thirdsheet of material on said one of said first and second sheets ofmaterial, such that after the lamination of said first and secondsheets, said thin film defines a material layer between said first andsecond sheets that has an opening that is not smaller than saiddiaphragm, whereby said restriction is defined by a first portion ofsaid at least two slits that extends through said first sheet ofmaterial and a second portion that extends under said rear side of saiddiaphragm and is defined by said material layer and an edge of saidsecond sheet of material that defines said recess.
 16. The methodaccording to claim 13, further comprising prior to the lamination ofsaid first and second sheets of material, etching said first sheet ofmaterial on its rear side such that after the lamination of said firstand second sheets of material said restriction is defined by a firstportion of said at least two slits that extends through said first sheetof material and a second portion that extends under said rear side ofsaid diaphragm.
 17. The method according to claim 13, wherein forming atleast two elongate narrow slits completely through a thin first sheet ofmaterial comprises simultaneously forming at least two elongate narrowslits completely through each of a plurality of portions of said firstsheet of material, such that said slits define in each of said pluralityof portions of said first sheet of material a restriction and divideeach of said plurality of portions into (i) a frame, (ii) a diaphragmattached to said frame and separated from said frame by said slits, withsaid diaphragm having an outer edge, a measurement side that is to facea surrounding environment, and a rear side, (iii) a first attachmentstructure interconnecting said frame and a first section of said outeredge of said diaphragm, and (iv) a second attachment structureinterconnecting said frame and a second section of said outer edge ofsaid diaphragm; wherein forming a recess in a second sheet of materialcomprises simultaneously forming a recess in a plurality of portions ofsaid second sheet of material, wherein laminating said first sheet ofmaterial to said second sheet of material comprises laminating saidfirst sheet of material to said second sheet of material such thatcorresponding ones of said plurality of portions of said first sheet ofmaterial become bonded to corresponding ones of said plurality ofportions of said second sheet of material, whereby said recess of eachof said plurality of portions of said second sheet of material defines areference chamber that faces said rear side of a corresponding saiddiaphragm and is in fluid communication with a corresponding saidrestriction, and further comprising sectioning said laminated first andsecond sheets of material into a plurality of individual said pressuresensors.
 18. A method for manufacturing a photoacoustical gas detectionsensor for detection and/or concentration determination of a particulargas in a surrounding environment, comprising: forming at least twoelongate narrow slits completely through a thin first sheet of materialsuch that said slits define a restriction and divide said first sheet ofmaterial into (i) a frame, (ii) a diaphragm attached to said frame andseparated from said frame by said at least two slits, with saiddiaphragm having an outer edge, a measurement side and a rear side,(iii) a first attachment structure interconnecting said frame and afirst section of said outer edge of said diaphragm, and (iv) a secondattachment structure interconnecting said frame and a second section ofsaid outer edge of said diaphragm; forming a first recess in a secondsheet of material; laminating said first sheet of material to saidsecond sheet of material such that said first recess defines ameasurement chamber that faces said measurement side of said diaphragmand is in fluid communication with said restriction, with saidmeasurement chamber having a wall area or window; forming a secondrecess in a third sheet of material; laminating said third sheet ofmaterial to said first sheet of material in an atmosphere of saidparticular gas such that said second recess defines a reference chamberthat faces said rear side of said diaphragm and is in fluidcommunication with said restriction, whereby said measurement chamberand said reference chamber are in fluid communication with one anothervia said restriction such that said particular gas is present in each ofsaid reference chamber and said measurement chamber; providing aradiation source that is spaced from said measurement chamber, with saidradiation source to emit radiation to which said wall area or window ofsaid measurement chamber is transparent, and with said radiation havinga wavelength that can be one of absorbed and scattered by saidparticular gas; and providing a signal-generating element to detectmechanical stress in at least one of said first attachment structure andsaid second attachment structure, wherein at least one of said firstattachment structure and said second attachment structure provides areduced area where mechanical stresses caused by pressure areconcentrated, and the provision of said signal-generating elementincludes arranging said signal-generating element at said reduced area,and wherein a combination of said frame, said reference chamber, saiddiaphragm, said first attachment structure, said second attachmentstructure, said restriction and said signal-generating elementconstitute a pressure sensor to measure absolute dynamic pressure insaid measurement chamber.
 19. The method according to claim 18, whereinsaid first sheet of material and said second sheet of material eachcomprise a sheet of silicon, said third sheet of material comprises asheet of glass, the formation of said at least two slits in said firstsheet of material comprises ionic etching said first sheet of material,the formation of said first recess in said second sheet of materialcomprises one of wet etching and machining said second sheet ofmaterial, and the formation of said second recess in said third sheet ofmaterial comprises one of wet etching and machining said third sheet ofmaterial.
 20. The method according to claim 19, further comprisingproviding said second sheet of material with a through hole by one ofmachining and wet etching said second sheet of material, and laminatinga fourth sheet of material to said second sheet of material such thatsaid through hole is covered by said fourth sheet of material, with saidfourth sheet of material being transparent to the radiation and definingsaid wall area or window of said measurement chamber.
 21. The methodaccording to claim 19, wherein forming at least two elongate narrowslits completely through a thin first sheet of material comprisessimultaneously forming at least two elongate narrow slits completelythrough each of a plurality of portions of said first sheet of material,such that said slits define in each of said plurality of portions ofsaid first sheet of material a restriction and divide each of saidplurality of portions into (i) a frame, (ii) a diaphragm attached tosaid frame and separated from said frame by said slits, with saiddiaphragm having an outer edge, a measurement side and a rear side,(iii) a first attachment structure interconnecting said frame and afirst section of said outer edge of said diaphragm, and (iv) a secondattachment structure interconnecting said frame and a second section ofsaid outer edge of said diaphragm; wherein forming a first recess in asecond sheet of material comprises simultaneously forming a recess in aplurality of portions of said second sheet of material, wherein forminga second recess in a third sheet of material comprises simultaneouslyforming a recess in a plurality of portions of said third sheet ofmaterial, wherein laminating said first sheet of material to said secondsheet of material and laminating said first sheet of material to saidthird sheet of material comprises laminating said first sheet ofmaterial to said second sheet of material such that corresponding onesof said plurality of portions of said first sheet of material becomebonded to corresponding ones of said plurality of portions of saidsecond sheet of material while laminating said first sheet of materialto said third sheet of material such that corresponding ones of saidplurality of portions of said first sheet of material become bonded tocorresponding ones of said plurality of portions of said third sheet ofmaterial, whereby said first recess of each of said plurality ofportions of said second sheet of material defines a measurement chamberthat faces said measurement side of a corresponding said diaphragm andis in fluid communication with a corresponding said restriction, andsaid second recess of each of said plurality of portions of said thirdsheet of material defines a reference chamber that faces said rear sideof a corresponding said diaphragm and is in fluid communication with acorresponding said restriction and a corresponding said measurementchamber via said corresponding said restriction, and further comprisingsectioning said laminated first, second and third sheets of materialinto a plurality of individual said photoacoustical gas detectionsensors.